Magnetic recording/reproducing equipment for computers or the like generally drives a magnetic disk of the hard type having a magnetic thin film on a rigid substrate relative to a flying magnetic head for magnetic recording/reproducing purposes.
The flying magnetic head has a slider which generates floating forces in a air bearing fashion. The flying magnetic head is generally classified into the composite type in which a core is integrated with the slider and the monolithic type in which a core also serves as the slider.
For maximum density recording, flying thin film magnetic heads have been used in practice. The flying thin film magnetic heads are formed by forming magnetic pole, gap, coil and other necessary layers on a nonmagnetic base by vapor phase deposition techniques or wet plating techniques. In these flying thin film heads, the base plays the role of a slider.
Most magnetic disks used in the past were of the coating type. To meet a demand for increasing the capacity of magnetic disks, magnetic disks of the thin film type now find increasing use. The thin film type magnetic disks have a magnetic thin film, also known as a continuous thin film, which is formed by vapor phase deposition techniques such as sputtering. They have excellent magnetic properties and increased recording density.
In the coating type magnetic disks, surface properties of a substrate do not significantly affect surface properties of the disk medium since the magnetic layer is as thick as 1 to 2 .mu.m. In the thin film type magnetic disks, however, surface properties of a substrate do significantly affect surface properties of the disk medium since the magnetic layer is as thin as 0.5 .mu.m or less. Thus the surface properties of the disk medium may be improved by using a substrate having a high precision surface. Then the magnetic head can assume a reduced floating distance above the disk, resulting in a reduced spacing loss, which in turn ensures an increased output and recording density.
Certain thin film type magnetic disks use modified substrates. One known modification is by forming an Ni-P plating layer of about 50 .mu.m thick on an aluminum alloy substrate followed by polishing the plating surface. Also known are those substrates which are prepared by anodizing an aluminum alloy substrate on the surface to form a hard oxide layer of about 2 .mu.m thick followed by polishing the anodized surface. These modified substrates have a surface roughness Rmax of about 0.15 .mu.m.
Magnetic thin films are formed on such substrates. For example, a magnetic thin film consisting essentially of Co-Ni may be formed by sputtering Cr on the substrate, further sputtering a Co-Ni base alloy thereon to form a thin film of about 1,000 A thick, and depositing a protective/lubricating coating of carbon or the like to a thickness of about 200 .ANG.. The surface of the resulting medium reflects the substrate surface properties and has a surface roughness Rmax of about 0.15 .mu.m.
Another typical magnetic thin film is one consisting essentially of iron oxide. A medium is prepared by sputtering an iron base target in an atmosphere of Ar plus O.sub.2 to form a sputtered film predominantly comprising .alpha.-Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4 on the substrate to a thickness of about 2,000 A. If the sputtered material is .alpha.-Fe.sub.2 O.sub.3, it is converted into Fe.sub.3 O.sub.4 by heating to a temperature of about 300.degree. C. in a reducing atmosphere. The next step is heating at a temperature of about 300.degree. C. in an oxidizing atmosphere for oxidizing Fe.sub.3 O.sub.4, forming a film predominantly comprising .gamma.-Fe.sub.2 O.sub.3. A protective/lubricating coating is deposited thereon to complete a medium. The resulting medium also has the surface which reflects the substrate surface properties, presenting a surface roughness Rmax of about 0.15 .mu.m.
Now that the entire medium manufacturing process is briefly described, the starting substrate is described again in detail. The aluminum alloy substrate having an Ni-P plating layer polished suffers from complexity of substrate preparation because the aluminum alloy substrate must be surface activated prior to Ni-P plating. The surface activation and subsequent steps are a main cause for adding to the substrate cost since these steps occupy more than 50% of the cost of the final substrate.
Further, the Ni-P plating layer tends to crystallize and thus possess magnetism when heated at temperatures of higher than 150.degree. C. The use of Ni-P plated aluminum alloy substrates is negated if the medium manufacturing process involves heating as in forming a magnetic thin film of iron oxide.
On the other hand, the anodized aluminum alloy substrates have the drawback that the anodized film tends to crack during heat treatment because stresses develop due to the differential thermal expansion between the aluminum alloy matrix and the anodized film. The heating temperature must be limited to below 300.degree. C. in forming a magnetic thin film of iron oxide. The anodized film is of a porous structure having a plurality of conductive pores. When a magnetic thin film is formed on such an anodized substrate, magnetic defects often occur at the sites of conductive pores. Also, the surface roughness of the anodized film is not satisfactory as shown by Rmax of about 0.15 .mu.m.
When the substrate has a relatively large surface roughness Rmax, the magnetic thin film formed thereon also has a correspondingly large surface roughness Rmax. If the spacing between the flying head and the disk in operation is reduced, there arises a possibility that the head can make contact with protuberances on the disk surface causing abrasion of the magnetic thin film or damages to the magnetic head.
The surface precision of a medium is a critical factor for allowing the magnetic head to float in a stable manner and for minimizing the floating distance between the head and the medium.
U.S. Pat. No. 3,516,860 discloses magnetic disks using glass substrates. The use of glass substrates is advantageous in that they remain intact upon heating in the magnetic thin film forming process. It is disclosed that the glass surface is preferably made as smooth as possible. However, no specific reference is made to the surface roughness of the glass substrate and magnetic thin film although the head can undesirably cling to the magnetic thin film if the film surface is too smooth.
More particularly, if the magnetic head had clung to the magnetic disk, the head could not quickly take off for floating at the start of rotation of the disk. The disk rotates while the head slider remains in contact with the magnetic thin film surface. This results in failure of the magnetic thin film and the head slider. If the magnetic head firmly clings to the magnetic disk, the head slider continues close or immovable contact with the magnetic thin film surface, disabling the disk from starting rotation.
Therefore, clinging is a serious failure by nature and adversely affects CSS durability as will be described later.
The inventors proposed in Japanese Patent Application Kokai No. 43819/1987 a magnetic disk in which a magnetic thin film is formed on a glass substrate having a precision finished surface. More specifically, the glass substrate is precision processed on the surface to a surface roughness Rmax of up to 100.ANG., preferably up to 50 .ANG.. The glass substrate may be at least partially tempered. No reference is made to the lower limit of the surface roughness Rmax of the glass substrate or to the range of surface roughness Rmax of the medium or magnetic thin film. There are disclosed two examples in which glass substrates have a surface roughness Rmax of 90 .ANG. and 40 .ANG. and magnetic thin films have a surface roughness Rmax of 100 .ANG. and 45 .ANG.. When these disks are used, the spacing of the flying head from the disk can be reduced to about 0.1 to about 0.2 .mu.m.
The inventors found that head clinging can occur when the magnetic thin film has a surface roughness Rmax of 40 A or less. Head clinging cannot sometimes be prevented if the spacing of the flying head from the disk is reduced to about 0.1 .mu.m or less. Repeating a contact-start-and-stop (CSS) cycle on a disk with the spacing of the flying head from the disk set to 0.2 .mu.m or less, we observed an output drop after a certain number of CSS cycles, indicating that the disk is still insufficient in durability. The CSS durability is particularly low at low temperatures of about 5.degree. to 15.degree. C. or at a flying head-to-disk spacing of 0.1 .mu.m or less.
We have found that the CSS durability of a magnetic disk can be significantly improved when both the surface roughnesses Rmax of a glass substrate and a magnetic thin film thereon are controlled to specific ranges.